How to automate high resolution renders in bulk
Rendering at 8k should not be problem for blender.

Rendering several small samples first is not faster than just rendering the whole thing. 

A solution for your large amount of 8k renderings could be sheepit https://www.sheepit-renderfarm.com/index.php you can use it to have other people render your projects, or you could check the setting that allows only you to render it. All you have to do is upload your blend files and click a few buttons. (I'm not going to provide a step by step tutorial because there are thousands of easy to find tutorials on youtube like this one https://www.youtube.com/watch?v=0M8hO6k8Kqs)